The present invention relates to total nitrogen oxide (NOX) measurement systems for use in harsh environments. The present invention relates to a nitrogen oxide (NOX) measurement system having a platinum and zeolite based catalyst filter and a sensor element having a potential which varies in response to a NOX component in a gas being measured.
There is a continuing need for high temperature NOx sensors for combustion environments due to government regulations and negative effects on ecosystems and health. The two main types of sensors that have been tested for NOx are the solid electrolyte (potentiometric and amperometric) and semiconducting types. One of the main drawbacks of these sensors that has hindered their development is the lack of selectivity between the two main NOx components of interest, NO and NO2. In combustion environments NO is often the dominant NOx species with NO2 being present to a lesser amount and it would be ideal to have a selective sensor for each. However, the majority of ceramic sensors cannot distinguish between the two species giving a signal response to both NO and NO2. Typically the signals are in opposite directions, although there are some sensors where the NO and NO2 signal was shown to go in the same direction. Nevertheless because the sensors respond to both gases it would be difficult to determine the level of NO and NO2 in a mixture. The majority of reports do not test mixtures of NO and NO2 together, which is most likely because the signal due to NO+NO2 would be less than that of NO or NO2 tested separately because of cancellation effects. In certain studies of solid electrolyte sensors they have been made to be semi-selective to NO or NO2 by polarizing the sensor electrodes.
Another approach has been to develop systems to detect total NOx, which would be a signal due to the sum of NO+NO2. One of the methods proposed to do this has been to build a two-chamber device out of yttria-stabilized zirconia (YSZ) and in the first chamber to electrochemically oxidize all the incoming NOx gas to NO2 and then detect the NO2 as a xe2x80x9ctotal NOxxe2x80x9d signal in the second chamber. This method and its variations have been extensively represented in the patent literature. A second method to detect total NOx has been to use a chemical or catalytic filter placed before the sensor to alter the incoming gas. For example, materials used as chemical filters such as Mo converters can convert all the NOx to NO under certain conditions and KMnO4 was shown to partially oxidize NO to NO2 but the disadvantage is that they are both consumed over time and have to be replaced.
Catalytic filters equilibrate the incoming NOx to a thermodynamically defined ratio depending on the oxygen content of the gas and the temperature. The advantage of the catalytic filter is that it is not consumed in the reaction. The use of a NOx equilibration catalytic filter before a sensor has the advantage of simplicity and longer life. A Pt-SiO2/WO3 catalyst layer was used on an amperometric design to equilibrate NOx to NO2 at 150xc2x0 C. but the effect of higher temperatures was not investigated. Some other catalytic filter materials that have been tested for NOx equilibration at various temperatures for possible sensor use are Pt black catalyst, Pt on cordierite, Mn3O4, Co3O4 and Pt on Al2O3.
The use of zeolites as a sensor filter for alcohols has been shown before. The zeolite""s own properties can be used to transform the incoming gas or it can be used as a support for an additional catalyst. It has also been shown before that gases such as CO, which is also present in a combustion environment, can interfere with the signal for NOx. Thus to measure an accurate level of NOx the CO cross-sensitivity must be minimized. Our approach in this study was to develop a system that could detect the total NOx gas concentration in a background of O2 and N2 at high temperatures with minimal CO interference. We used a non-selective YSZ air reference sensor to detect the NOx and a NOx equilibration/CO oxidation filter placed before the sensing electrode composed of a Pt catalyst dispersed onto a zeolite Y support. The sensor and the filter were maintained at different temperatures to provide a driving force for the NOx equilibration reactions.
The present invention presents a novel measurement system for determining total NOX concentration, from a gas sample. Total NOX includes pure NO, pure NO2 and mixtures thereof. The measurement system comprises a gas conduit having an upstream end and a downstream end. The gas conduit carries a gas comprising NOX. The gas introduced into a measurement system of the present invention typically has concentrations of NO, NO2 and CO in the range of 0 to 1000 ppm. Further, the gas typically contains 2 to 3% oxygen (O2). Disposed within the gas conduit is a catalyst filter comprising platinum and a zeolite. The gas flowing through the gas conduit interacts with the catalyst filter at a particular temperature to form an equilibrium mixture of NO and NO2 from the gas comprising NOX. The measurement system further comprises a sensor element having two electrodes on a solid electrolyte yttria-stabilized zirconia; a sensing potentiometric electrode disposed downstream of the catalytic filter device so as to contact the equilibrium mixture of NO and NO2 and a reference potentiometric electrode. Typically, the reference potential electrode is referenced to air.
It is preferred that the catalyst filter contain between 1 to 5% by mass of platinum. Further, it preferred that zeolite Y be used as the zeolite.
It is preferred that the catalytic filter be placed in the gas conduit so as to maximize exposure of the catalyst filter to the gas stream, thereby better effectuating the equilibrium formation of NO and NO2. Further, it is preferred that the catalyst filter be temperature controlled. Heating of the catalytic filter may be accomplished by any conventional means. By adjusting the temperature of the catalytic filter, the equilibrium concentration of NO and NO2 can be adjusted. It is most preferred that the temperature of the catalytic filter be maintained at a temperature below approximately 700xc2x0 C., to avoid decomposition of the zeolite.
It is preferred that yttria-stabilized-zirconia be used as the solid electrolyte. It is also preferred that the reference potentiometric electrode is constructed of platinum. It is further preferred that the sensing potentiometric electrode is constructed from platinum, chromium oxide or cobalt oxide. It is preferred that the sensor element be temperature controlled. Temperature control of the sensor element may be accomplished by any conventional means. It is most preferred that the electrolyte, along with the sensing potentiometric electrode and the reference potentiometric electrode are maintained at a temperature above approximately 400xc2x0 C. and below approximately 600xc2x0 C. Finally, it is preferred that the temperature of the catalytic filter is maintained at a different temperature than the temperature of the sensor element to improve the magnitude of the signal. The greater the temperature difference between the catalytic filter and the sensor element, the larger the magnitude of the signal. It is most preferred to have at least a 100xc2x0 C. temperature difference between the catalytic filter and the sensor element. Additionally, it is preferred that the temperatures of the catalytic filter and the sensor element be known in order to establish the calibration curve for the measurement system.
A method of determining the total NOX content in a gas of the present invention comprises: (a) exposing the gas comprising NOX to a catalytic filter comprising platinum and a zeolite for a sufficient time so as to form an equilibrium mixture of NO and NO2 from the gas comprising NOX, (b) exposing the equilibrium mixture of NO and NO2 to a sensor element comprising a sensing potentiometric electrode and a reference potentiometric electrode each disposed on an electrolyte substrate so as to obtain a potential difference between the two electrodes, and (c) determining the total NOX content in the gas comprising NOX from the potential difference via a calibration curve.
Using the present invention, the total NOX content may be determined from a gas at any temperature, including those in high temperature exhaust streams in the range of about 400xc2x0 C. to about 600xc2x0 C. The gas comprising NOX may additionally comprise other gases such as carbon monoxide and oxygen. It is preferred that any oxygen present be at a substantially constant concentration because the measurement system employs a calibration curve for NOX concentration determination that is oxygen sensitive. However, minor variation in the oxygen concentration will not cause substantial error.
A second method for determining the total NOX content in a gas comprising NOX of the present invention comprises: conducting the gas comprising NOX through a gas conduit having an upstream end and a downstream end. The gas conduit has disposed therein a catalytic filter comprising platinum and a zeolite. The catalytic filter catalyzes the gas to form an equilibrium mixture of NO and NO2. The gas then passes over a sensor element comprising a sensing potentiometric electrode and a reference potentiometric electrode each on a solid electrolyte downstream of the catalytic filter where the potential difference between the sensing potentiometric electrode and the reference potentiometric electrode is measured. The total NOX content is then determined by comparing the potential difference with a calibration curve.